1. Field of the Invention
The present invention relates to a burner for decomposing nonflammable materials, and in particular, to a burner which is suited for use in thermally decomposing, at high efficiency, PFC (perfluorocarbon), etc. which are included in a process exhaust gas-to be discharged from a semiconductor manufacturing process.
2. Description of the Related Art
The process exhaust gas to be discharged from a semiconductor manufacturing process is known to contain various kinds of harmful gas components. Therefore, there have been proposed various kinds of exhaust gas disposing apparatus with an aim to treat such harmful exhaust gases before the exhaust gas is discharged into the external atmosphere. For example, Japanese Patent Unexamined Publication (Kokai) No.2001-165422 describes an exhaust gas-processing apparatus which is designed to effectively decompose PFC such as C2F6 which is a nonflammable gas. This exhaust gas-processing apparatus is constructed such that a plurality of fuel gas combustion nozzles are disposed in multi-stages in a vertically disposed cylindrical body and in such a manner that the flame-ejecting direction of each of the fuel gas combustion nozzles is inclined relative to the plane orthogonally intersecting with the central axis of the cylindrical body. Due to this structure, the residence time of exhaust gas can be prolonged in this wide combustion region of high-temperature, thereby promoting the thermal decomposition of nonflammable gas, thus reforming the nonflammable material-containing gas.
Further, Japanese Patent Unexamined Publication (Kokai) No.2001-280629 describes a combustion type exhaust gas-processing apparatus which comprises: a combustion cylinder which is constituted by an outer cylinder and an inner cylinder; an exhaust gas combustion nozzle which is provided with an exhaust gas passageway and with a fuel gas supply passageway disposed around and coaxial with the exhaust gas passageway and disposed at the bottom portion of the inner cylinder; and means for feeding a combustion-sustaining gas pressurized higher than atmospheric pressure into the inner cylinder. According to this apparatus, flame is created so as to surround the exhaust gas passageway, and the combustion-sustaining gas pressurized higher than atmospheric pressure is blown into the flame, thereby easily thermally decomposing nonflammable and harmful components such as nonflammable PFC in the exhaust gas.
It has been found, through repeated experiments and studies made by the present inventors in an effort to effectively decompose the exhaust gas containing nonflammable materials such as a process exhaust gas to be discharged from a semiconductor manufacturing process, that all of the aforementioned processing apparatuses that have been conventionally proposed are rather complicated in structure and are inevitably caused to become larger in size, thus necessitating the development of a smaller processing apparatus in view of limited environmental space available in manufacturing industries. It has been also found, through repeated experiments and studies made by the present inventors, that although these conventional processing apparatuses are effective in achieving a high processing efficiency with respect to the materials such as C2F6 that can be thermally decomposed at a relatively low temperature, it is not necessarily possible to obtain a high processing efficiency with respect to CF4 or SF6 which are relatively high in thermal decomposition temperature.
The present invention has been made to cope with the aforementioned circumstances, and therefore an object of the present invention is to provide a burner for decomposing nonflammable materials, which is simple in structure and capable of thermally decomposing even a material having a relatively high thermal decomposition temperature such as CE4 or SF6 at as high efficiency as 99% or more.
With a view to solve the aforementioned problems, there is provided, according to the present invention, a burner for decomposing nonflammable materials, which comprises: a cylindrical body having one end thereof closed with a blocking wall; a nonflammable material-containing gas-introducing nozzle which is secured to the blocking wall in a manner to permit the nonflammable material-containing gas to be injected around the central axis of the cylindrical body; and a plurality of oxidizing agent/fuel blow-off nozzles which are attached to said blocking wall in a manner that these nozzles are positioned on and along circular lines which are coaxial with the central axis of the cylindrical body; wherein the plurality of oxidizing agent/fuel blow-off nozzles are inclined in such a degree as to enable flame ejected from said plurality of oxidizing agent/fuel blow-off nozzles to converge onto approximately the same point on the central axis of said cylindrical body.
Since the burner for decomposing nonflammable materials according to the present invention is simply provided with a cylindrical body, a nonflammable material-containing gas-introducing nozzle, and a plurality of oxidizing agent/fuel blow-off nozzles which are positioned on and along circular lines which are coaxial with the central axis of the cylindrical body, the burner is very simple in structure and can be designed relatively short in overall length. Therefore, it is now possible to easily install a process exhaust gas processing installation below the floor of a semiconductor manufacturing plant for instance.
Further, since the flame ejected from a plurality of oxidizing agent/fuel blow-off nozzles which are positioned coaxial with the nonflammable material-containing gas-introducing nozzle is enabled to converge onto approximately the same point on the central axis of the cylindrical body, a high-temperature combustion region can be formed around the converged region of the flame. Furthermore, since the nonflammable material-containing gas or a process exhaust gas discharged from a semiconductor manufacturing apparatus for instance is caused to pass through this high-temperature combustion region without fail, the thermal decomposition process of nonflammable materials such as PFC can be effectively proceeded. According to the experiments performed by the present inventors, it was found possible to achieve the decomposition of CF4 which is high in decomposition temperature at a decomposition ratio of 99% or more, and even with respect to SF6, it was possible to obtain almost the same decomposition ratio as that of CF4. With respect to other kinds of PFC such as C2F6 which are relatively low in thermal decomposition temperature as compared with CF4 and SF6, it is possible to achieve a high thermal decomposition ratio.
In the case of the burner for decomposing nonflammable materials according to the present invention, an optimal angle of the inclination of the axis of each of said oxidizing agent/fuel blow-off nozzles relative to the central axis of the cylindrical body may be changed depending on the flow rate or velocity of the nonflammable material-containing gas to be ejected from the nonflammable material-containing gas-introducing nozzle, on the size of the flame ejected from the oxidizing agent/fuel blow-off nozzles, or on the distance between the oxidizing agent/fuel blow-off nozzles and the tip end of the flame. Although the optimal value of the aforementioned inclination angle can be experimentally determined under given conditions, it has been determined through experiments made by the present inventors that the axis of each of the oxidizing agent/fuel blow-off nozzles should preferably be inclined, in practical viewpoint, at an angle ranging from 15 to 50 degrees, more preferably from 30 to 45 degrees relative to the central axis of the cylindrical body. If this inclination angle is larger than 50 degrees, the flame may come too close to the rear side of blocking wall, resulting in the acceleration of thermal damage of the refractories constituting the blocking wall. On the other hand, if this inclination angle is smaller than 15 degrees, the flame may come too close to the inner wall of the cylindrical body, resulting also in the acceleration of thermal damage of the refractories constituting the cylindrical body, and at the same time, making it difficult to sufficiently entrain the flame into the flux of the nonflammable material-containing gas, thereby raising various problems such as the deterioration of the decomposition ratio.
It is desirable, on the occasion of operating the burner for decomposing nonflammable materials according to the present invention, to retard the flow velocity of the nonflammable material-containing gas that will be introduced into the cylindrical body from the nonflammable material-containing gas-introducing nozzle. When the flow velocity of the nonflammable material-containing gas is retarded in this manner, the residence time of the nonflammable material-containing gas in the high-temperature combustion region would be prolonged, thereby making it possible to obtain high thermal decomposition efficiency. In the case where the flow rate is kept constant, the flow velocity of the gas is caused to vary depending on the diameter of the nonflammable material-containing gas-introducing nozzle, so that it is more desirable to increase the diameter of the nozzle as long as it is permissible in environmental viewpoint. Additionally, if the flow velocity of the nonflammable material-containing gas is increased, the nonflammable material-containing gas may be caused to pass through the flame in such a manner that the flame is thrust aside, thereby deteriorating the thermal decomposition of the gas. Therefore, the optimal value of the flow velocity (the diameter of the nonflammable material-containing gas-introducing nozzle) should be experimentally set taking the aforementioned circumstances into consideration.